1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display device. The present invention relates to, for example, a method for manufacturing an electro-optical device typified by a liquid crystal display panel which has a circuit including a thin film transistor (hereinafter referred to as a TFT), and a method for manufacturing an electronic device equipped with such an electro-optical device as a component.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (with a thickness of about several nm to several hundreds nm) which is formed over a substrate having an insulating surface has attracted attention. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device, and has been hurriedly developed particularly as a switching element of an image display device.
A liquid crystal display device is known as an image display device. Active matrix liquid crystal display devices have been commonly used because a high-definition image can be obtained as compared with the case of a passive matrix liquid crystal display device. In the active matrix liquid crystal display device, an image is displayed on a screen by driving pixel electrodes arranged in matrix. Specifically, by applying a voltage between a selected pixel electrode and an opposite electrode that faces the pixel electrode, optical modulation of a liquid crystal layer interposed between the pixel electrode and the opposite electrode is performed, and this optical modulation is recognized as an image.
The range of application of such an active matrix electro-optical device is increased, and demands for high definition, high aperture ratio, and high reliability have been increasing in accordance with increase of screen size. At the same time, demands for progress in productivity and cost reduction have also been increasing.
In addition, as panel size increases, the cost of a material to be used is increased. In particular, a liquid crystal material interposed between a pixel electrode and an opposite electrode is expensive.
In the case where a liquid crystal injection method is used, sealing of liquid crystals requires a complicated process of seal drawing, attachment of an opposite substrate, cutting, injection of liquid crystals, sealing of an inlet for injecting liquid crystals, and/or the like. In particular, as the panel size increases, it becomes difficult to fill a region (including at least a pixel portion) surrounded by a sealant with liquid crystals because liquid crystal injection is performed using a capillary phenomenon. Further, when liquid crystal injection is performed using a capillary phenomenon, liquid crystals which are more than liquid crystals which are injected into the inlet for injecting liquid crystals is used and wasteful liquid crystals occur.
In addition, in the case where the liquid crystal injection method is used, in which two substrates are attached, cutting is performed thereon, and a liquid crystal material is injected through the inlet for injecting liquid crystals, which is formed in the cut surface. However, a portion serving as a path of the liquid crystal material, which extends from the inlet for injecting liquid crystals to a pixel region, is also filled with liquid crystals. As described above, a portion other than the region for forming a display portion is also filled with the liquid crystal material.
In addition, a portion of the path of the liquid crystal material, which extends from the inlet for injecting liquid crystals to the pixel region, particularly, near the inlet for injecting liquid crystals, is a portion through which liquid crystals which are extremely more than through the other portion of a panel pass, and friction generated at the time of injection changes the surface of an alignment film, which may cause disorder of orientation of liquid crystals.
In addition, in the liquid crystal injection method, a step of sealing the inlet for injecting liquid crystals after injection of liquid crystals is needed.
The present applicant has proposed a technique in which liquid crystals are dropped and then a pair of substrates is attached to each other under reduced pressure, as a method for manufacturing a liquid crystal display device without using a liquid crystal injection method, in Patent Document 1 (U.S. Pat. No. 4,691,995).
Further, the present applicant has proposed a technique in which a conductor for connection to an opposite electrode is disposed inside of a sealing material and a barrier wall is provided so as to surround the conductor in Patent Document 2 (Japanese Published Patent Application No. 2006-268020).
The technology disclosed in Patent Document 1 is called a liquid crystal dropping method (ODF: one drop fill). The liquid crystal dropping method can eliminate material loss because liquid crystals as many as necessary are dropped only in a necessary portion. In addition, since a seal pattern has a closed-loop shape, an inlet for injecting liquid crystals is not needed. In addition, a defect caused by change of the surface of an alignment film in a path of liquid crystals (e.g., defective orientation) can be eliminated.
Further, a process sequence of the liquid crystal dropping method is largely different from that of the liquid crystal injection method.
A procedure for manufacturing a liquid crystal display device by the liquid crystal injection method is explained below. First, a sealing material is drawn on an opposite substrate by a screen printing method or with a dispensing apparatus. Next, the opposite substrate and another substrate are disposed so as to face each other, the sealing material is hardened, and the substrates are attached to each other. Next, the pair of substrates is cut such that part of the sealing material (an inlet for injecting liquid crystals) is located in an edge surface of the substrates. Next, the pair of substrates is disposed in a chamber where the pressure is reduced, and the pressure inside the chamber is gradually returned from reduced pressure to atmospheric pressure while a liquid crystal material is in contact with the inlet for injecting liquid crystals, whereby liquid crystals are injected using a capillary phenomenon through the inlet for injecting liquid crystals. Next, the inlet for injecting liquid crystals is sealed with a sealant, and the sealant is hardened by ultraviolet irradiation. Lastly, heat treatment for aligning the orientation of liquid crystals is performed thereon.
Next, a procedure for manufacturing a liquid crystal display device by the liquid crystal dropping method is explained below. First, a closed-loop pattern of a sealing material is drawn on an opposite substrate with a dispensing apparatus. Next, liquid crystals are dropped in a region surrounded by the sealing material on the opposite substrate by a desired amount. Next, the opposite substrate and another substrate are attached to each other under reduced pressure. Next, the pressure of the atmosphere surrounding the pair of substrates is changed from reduced pressure to atmospheric pressure. Next, ultraviolet irradiation is performed thereon to harden the sealing material. Next, heat treatment for further hardening the sealing material and heat treatment for aligning the orientation of liquid crystals are performed at the same time. Lastly, the pair of substrates is cut.
In the liquid crystal injection method, a pair of substrates are attached and cut, and then liquid crystals are injected; whereas, in the liquid crystal dropping method, liquid crystals are dropped on one of a pair of substrates, the pair of substrates is attached to each other under reduced pressure, and then the pair of substrates is cut.
Further, as for the heat treatment which is needed to align the orientation of liquid crystals, heat treatment is performed to align the orientation of liquid crystals after a sealant is hardened in the liquid crystal injection method. On the other hand, in the liquid crystal dropping method, heat treatment for hardening a sealing material and heat treatment for aligning the orientation of liquid crystals are performed at the same time; thus a liquid crystal display device is manufactured efficiently.